package org.apache.lucene.index;

import java.io.IOException;

import org.apache.lucene.store.Directory;

/**
 * This stores a monotonically increasing set of <Term, TermInfo> pairs in a
 * Directory.  Pairs are accessed either by Term or by ordinal position the
 * set.
 */
final class TermInfosReader {
  private Directory directory;
  private String segment;
  private FieldInfos fieldInfos;

  private ThreadLocal enumerators = new ThreadLocal();
  private SegmentTermEnum origEnum;
  private long size;

  TermInfosReader(Directory dir, String seg, FieldInfos fis) throws IOException {
    directory = dir;
    segment = seg;
    fieldInfos = fis;

    origEnum = new SegmentTermEnum(directory.openFile(segment + ".tis"), fieldInfos, false);
    size = origEnum.size;
    readIndex();
  }

  public int getSkipInterval() {
    return origEnum.skipInterval;
  }

  final void close() throws IOException {
    if (origEnum != null)
      origEnum.close();
  }

  /**
   * Returns the number of term/value pairs in the set.
   */
  final long size() {
    return size;
  }

  private SegmentTermEnum getEnum() {
    SegmentTermEnum termEnum = (SegmentTermEnum) enumerators.get();
    if (termEnum == null) {
      termEnum = terms();
      enumerators.set(termEnum);
    }
    return termEnum;
  }

  Term[] indexTerms = null;
  TermInfo[] indexInfos;
  long[] indexPointers;

  private final void readIndex() throws IOException {
    SegmentTermEnum indexEnum = new SegmentTermEnum(directory.openFile(segment + ".tii"), fieldInfos, true);
    try {
      int indexSize = (int) indexEnum.size;
      indexTerms = new Term[indexSize];
      indexInfos = new TermInfo[indexSize];
      indexPointers = new long[indexSize];

      for (int i = 0; indexEnum.next(); i++) {
        indexTerms[i] = indexEnum.term();
        indexInfos[i] = indexEnum.termInfo();
        indexPointers[i] = indexEnum.indexPointer;
      }
    } finally {
      indexEnum.close();
    }
  }

  /**
   * Returns the offset of the greatest index entry which is less than or equal to term.
   */
  private final int getIndexOffset(Term term) throws IOException {
    int lo = 0;            // binary search indexTerms[]
    int hi = indexTerms.length - 1;

    while (hi >= lo) {
      int mid = (lo + hi) >> 1;
      int delta = term.compareTo(indexTerms[mid]);
      if (delta < 0)
        hi = mid - 1;
      else if (delta > 0)
        lo = mid + 1;
      else
        return mid;
    }
    return hi;
  }

  private final void seekEnum(int indexOffset) throws IOException {
    getEnum().seek(indexPointers[indexOffset],
      (indexOffset * getEnum().indexInterval) - 1,
      indexTerms[indexOffset], indexInfos[indexOffset]);
  }

  /**
   * Returns the TermInfo for a Term in the set, or null.
   */
  TermInfo get(Term term) throws IOException {
    if (size == 0) return null;

    // optimize sequential access: first try scanning cached enum w/o seeking
    SegmentTermEnum enumerator = getEnum();
    if (enumerator.term() != null                 // term is at or past current
      && ((enumerator.prev != null && term.compareTo(enumerator.prev) > 0)
      || term.compareTo(enumerator.term()) >= 0)) {
      int enumOffset = (int) (enumerator.position / enumerator.indexInterval) + 1;
      if (indexTerms.length == enumOffset    // but before end of block
        || term.compareTo(indexTerms[enumOffset]) < 0)
        return scanEnum(term);        // no need to seek
    }

    // random-access: must seek
    seekEnum(getIndexOffset(term));
    return scanEnum(term);
  }

  /**
   * Scans within block for matching term.
   */
  private final TermInfo scanEnum(Term term) throws IOException {
    SegmentTermEnum enumerator = getEnum();
    while (term.compareTo(enumerator.term()) > 0 && enumerator.next()) {
    }
    if (enumerator.term() != null && term.compareTo(enumerator.term()) == 0)
      return enumerator.termInfo();
    else
      return null;
  }

  /**
   * Returns the nth term in the set.
   */
  final Term get(int position) throws IOException {
    if (size == 0) return null;

    SegmentTermEnum enumerator = getEnum();
    if (enumerator != null && enumerator.term() != null &&
      position >= enumerator.position &&
      position < (enumerator.position + enumerator.indexInterval))
      return scanEnum(position);      // can avoid seek

    seekEnum(position / enumerator.indexInterval); // must seek
    return scanEnum(position);
  }

  private final Term scanEnum(int position) throws IOException {
    SegmentTermEnum enumerator = getEnum();
    while (enumerator.position < position)
      if (!enumerator.next())
        return null;

    return enumerator.term();
  }

  /**
   * Returns the position of a Term in the set or -1.
   */
  final long getPosition(Term term) throws IOException {
    if (size == 0) return -1;

    int indexOffset = getIndexOffset(term);
    seekEnum(indexOffset);

    SegmentTermEnum enumerator = getEnum();
    while (term.compareTo(enumerator.term()) > 0 && enumerator.next()) {
    }

    if (term.compareTo(enumerator.term()) == 0)
      return enumerator.position;
    else
      return -1;
  }

  /**
   * Returns an enumeration of all the Terms and TermInfos in the set.
   */
  public SegmentTermEnum terms() {
    return (SegmentTermEnum) origEnum.clone();
  }

  /**
   * Returns an enumeration of terms starting at or after the named term.
   */
  public SegmentTermEnum terms(Term term) throws IOException {
    get(term);
    return (SegmentTermEnum) getEnum().clone();
  }
}
